1. Field of Invention
This invention relates to a new method and means for the dynamic correction and minimization of aberrations produced in the electron beam of electron beam tubes employing electrostatic deflection systems.
2. Prior Art Problems
In recent years considerable effort has been expended in the development of electron beam addressable memories for use as peripheral, add-on or main memories for computer systems. These memories, known as EBAM have been described in a number of publications and patents such as U.S. Pat. No. 3,886,530--issued May 27, 1975--Huber, et al., entitled "Signal Storage Device." In an electron beam addressable memory the number of data storage sites that the electron optical system can resolve at the target plane (at fixed current density), or the current density that can be achieved (with a fixed number of data bit sites), varies inversely with the electron beam spot aberration at the target plane. Electron beam spot aberration is introduced by the deflector system as it causes the electron beam to traverse from a center axis position across the x-y plane of a target surface to a particular address bit site location whose x-y coordinates identify the data to be stored and/or retrieved. For maximum data storage on a given target surface area, electron beam spot aberration must be kept to a minimum.
While the problem of spot aberration can be overcome through the use of magnetic deflection, which can be designed to have low aberration, magnetic deflection suffers from poor speed of response and lack of reproducibility (due to eddy current and hysteresis effects). To obtain high speed of response and good reproducibility, electrostatic deflection is preferred. However, known electrostatic deflection systems introduce significant beam spot aberration. Thus, a low-aberration electrostatic deflection system is needed for high speed, high resolution applications such as electron beam addressable memories and electron beam microfabrication systems.